The present invention relates to improvements in microwave power amplifiers.
Modulations presently in use in radio links, essentially Quadrature Amplitude Modulation (QAM), impose very stringent linearity requirements for the radio frequency power amplifier of the transmitter on which depends not a little the degradation of the modulated signal.
The power output from the final amplifier devices must be considerably lower than their saturation power so that the nonlinear distortions thereof introduced satisfy the specifications of the transmitter. These distortions are due to compression of gain at high power found in the trend of the Amplitude Modulation (AM/AM) distortion curve at high power and the amplitude modulation/phase modulation (AM/PM) conversion curve, again at high power.
Obviating these distortions usually involves oversizing these final amplifiers and hence high cost of the power amplifying section.
As known in itself, use of a linearizing network in the transmitting section permits use of power devices with lower saturation for a given distortion produced with a resulting increase of efficiency, e.g. for application in the transmitters of on-board repeaters in satellite communication systems or, for a given saturation power of final devices, such use also allows higher linearity of the amplifier, e.g. for applications in transmitters for earth stations in such satellite communication systems.
A linearization technique presently well known is termed "feed forward error control", and includes all the linearizers which use an auxiliary microwave amplifier which amplifies an error signal obtained by determining the difference between the input signal and the distorted one appropriately attenuated at the output from the main amplifier. The error signal is proportional to the distortions generated by the main amplifier so that, again added with appropriate phase and amplitude at the output of the main amplifier, this error signal reduces the distortions affecting the output signal.
It is clear that the merit figure or degree of quality of this linearization system depends almost exclusively on the balancing of the final adder or coupler which subtracts the error signal from the output signal of the amplifier. A balancing regulation circuitry (in amplitude and phase) of this coupler is therefore necessary and is quite complex. In addition, it is a true amplifier-linearizer complex in itself, not an addition to improve a known amplifier.
Another present linearization technique calls for the use of RF (radio frequency) predistorters, i.e. nonlinear networks inserted upstream of the final microwave amplifier, which distort the input signal by means of networks embodied with components which work in a nonlinear state in order to compensate for the AM/AM distortion curve and the amplitude/phase conversion curve AM/PM of the final power amplifier, and which guarantee better linearity of the transmitting section. The main drawback of these known predistorters consists, however, of the excessive complexity of said networks, and thus of their still excessive cost.
An example of a predistortion linearizer for microwave power amplifiers is described in Italian patent application No. 19497-A/87 filed by the same applicant Feb. 26, 1987, and incorporated herein.
In this previous Italian patent application there is described a predistortion linearizer applicable upstream of the power amplifier and which has a main network including a phase modulator and an amplitude modulator arranged in cascade. A secondary network with a base band frequency is provided. It includes means for amplitude detecting and filtering part of the input signal so as to produce a detected signal which is a function of the instantaneous input power. A pair of adjustable gain amplifiers are supplied with the detected signal and act on the modulators in such a manner as to give them nonlinear response curves, such as to compensate independently for both amplitude and phase nonlinearity of the power amplifier.
Such a linearizer, although it has all the advantages compared with the known art listed in the application, is still a costly and cumbersome embodiment, principally because of the presence therein of derived branches.